Field of the Invention
The present invention relates to a sheet feeding apparatus and to an image forming apparatus including the sheet feeding apparatus.
Description of the Related Art
A conventional image forming apparatus such as a printer, a copier, a facsimile includes a sheet feeding apparatus feeding a sheet to an image forming unit. The sheet feeding apparatus includes a pickup roller delivering the sheet stacked on a sheet supporting portion and a separating portion separating the sheet one by one in case the pickup roller delivers multiple sheets.
The separating portion includes a retard separating system in which a feed roller rotating in the same direction with a pickup roller and a retard roller in pressure contact with the feed roller with a predetermined pressure-contact force (nip pressure). It is noted that a drive of a certain torque (rotational torque), is applied to the retard roller in a direction reverse to a sheet conveyance direction (reverse direction) through a torque limiter, so that the retard roller can rotate in either direction of the sheet conveyance direction and the reverse direction.
The separating portion of the retard separating system prevents multiple feeding by rotating the retard roller in the reverse direction when two or more sheets enter a nip portion (separating nip portion), between the retard roller and the feed roller. The retard roller rotates following the feed roller when one sheet or no sheet enters the separating nip portion.
In order to improve stability in the sheet feeding operation, some sheet feeding apparatuses having the separating portion of the retard separating system are configured to switch the pickup roller between a separation state in which the pickup roller is separated from the sheet on the tray and an abutment state in which the pickup roller abuts with the sheet. In the case of a manual sheet feeding apparatus as an example of sheet feeding apparatus in particular, a user cannot set a sheet on a manual sheet feed tray in the abutment state. Therefore, it is necessary to switch the pickup roller to the separation state in setting the sheet.
Therefore, a conventional sheet feeding apparatus includes a lift mechanism lifting the pickup roller from an abutment position where the pickup roller is in contact with the sheet and to a separate position separated from the sheet. In setting the sheet, the pickup roller is moved to the separate state by the lift mechanism. As such a lift mechanism, U.S. Pat. No. 8,800,986 discloses a configuration using an idle gear interposed between the pickup roller and the feed roller. In this configuration, a corrugated washer is disposed between a holder liftably holding the pickup roller and the idle gear. Then, the holder turns by a frictional force generated by the corrugated washer when the idle gear rotates, and a pressing force pressing the pickup roller against the sheet is generated.
Japanese Patent Application Laid-open No. 2005-75479 also discloses another lift mechanism configured to lift a pickup roller through a cam rotated by a motor.
Here, since the contact pressure of the pickup roller against the sheet depends on the frictional force between the members in the lift mechanism described in U.S. Pat. No. 8,800,986, there is a possibility that the contact pressure is unstablized due to wear of frictional surfaces and to environmental conditions such as humidity.
In the case of the lift mechanism lifting the pickup roller by the cam driven by the motor and a cam follower actuated by the cam, it is possible to avoid such situation that the contact pressure of the pickup roller against the sheet fluctuates due to the friction and others. However, in the case of the lift mechanism using the cam and the cam follower, it has been difficult to dispose the cam and the cam follower in such a manner as to realize downsize of the apparatus. In the case of the manual sheet feeding apparatus, which is disposed in a narrow space in a side part of the image forming apparatus, in particular, the disposition of the cam and the cam follower is essential because the lift mechanism is also required to be downsized.